In HTML, the selected option is represented by the presence of the selected
attribute on the <option>
element like so:
<option ... selected>...</option>
Or if you're HTML/XHTML strict:
<option ... selected="selected">...</option>
Thus, you just have to let JSP/EL print it conditionally. Provided that you've prepared the selected department as follows:
request.setAttribute("selectedDept", selectedDept);
then this should do:
<select name="department">
<c:forEach var="item" items="${dept}">
<option value="${item.key}" ${item.key == selectedDept ? 'selected="selected"' : ''}>${item.value}</option>
</c:forEach>
</select>